--- ray/src/rt/pmaprand.h	2015/05/08 13:20:23	2.3
+++ ray/src/rt/pmaprand.h	2017/08/14 21:12:10	2.8
@@ -1,44 +1,52 @@
-#ifndef PMAPRAND_H
-#define PMAPRAND_H
+/* RCSid $Id: pmaprand.h,v 2.8 2017/08/14 21:12:10 rschregle Exp $ */
 
 /* 
-   ==================================================================
+   ======================================================================
    Random number generators for photon distribution
 
    Roland Schregle (roland.schregle@{hslu.ch, gmail.com})
    (c) Fraunhofer Institute for Solar Energy Systems,
    (c) Lucerne University of Applied Sciences and Arts,
-   supported by the Swiss National Science Foundation (SNSF, #147053)
-   ==================================================================
+       supported by the Swiss National Science Foundation (SNSF, #147053)
+   ======================================================================
    
-   $Id: pmaprand.h,v 2.3 2015/05/08 13:20:23 rschregle Exp $
+   $Id: pmaprand.h,v 2.8 2017/08/14 21:12:10 rschregle Exp $
 */
 
 
 
-/* According to the analytical validation, skipping numbers in the sequence
-   introduces bias in scenes with high reflectance. We therefore use
-   erand48() with separate states for photon emission, scattering, and
-   russian roulette. The pmapSeed() and pmapRandom() macros can be adapted
-   to other (better?) RNGs. */   
+#ifndef PMAPRAND_H
+   #define PMAPRAND_H
 
-#if defined(_WIN32) || defined(BSD)
-   /* Assume no erand48(), so use standard RNG without explicit multistate 
-      control; the resulting sequences will be suboptimal */      
-   #include "random.h"
-   
-   #define pmapSeed(seed, state) (srandom(seed))
-   #define pmapRandom(state)     (frandom())
+   /* According to the analytical validation, skipping sequential samples
+    * when sharing a single RNG among multiple sampling domains introduces
+    * overlapping photon rays (reported as 'duplicate keys' when building
+    * the underlying data structure) and therefore bias.  This is aggravated
+    * when running parallel photon distribution subprocesses, where photon
+    * rays from different subprocesses may correlate.    
+    * We therefore maintain a separate RNG state for each sampling domain
+    * (e.g. photon emission, scattering, and russian roulette).  With
+    * multiprocessing, each subprocess has its own instance of the RNG
+    * state, which is independently seeded for decorellation -- see
+    * distribPhotons() and distribPhotonContrib().    
+    * The pmapSeed() and pmapRandom() macros below can be adapted to
+    * platform-specific RNGs if necessary.  */
+#if defined(_WIN32) || defined(_WIN64)
+   /* Use standard RNG without state management; the generated sequences
+    * will be suboptimal */
+   #include "random.h"      
+   #define pmapSeed(seed, state) srandom(seed)
+   #define pmapRandom(state)     frandom()
 #else
+   /* Assume NIX and manage RNG state via erand48() */
    #define pmapSeed(seed, state) (state [0] += seed, state [1] += seed, \
                                   state [2] += seed)
-   #define pmapRandom(state) erand48(state)
+   #define pmapRandom(state)     erand48(state)
 #endif
 
-   
-extern unsigned short partState [3], emitState [3], cntState [3],
-                      mediumState [3], scatterState [3], rouletteState [3],
-                      randSeed;
-
+      
+   extern unsigned short partState [3], emitState [3], cntState [3],
+                         mediumState [3], scatterState [3], rouletteState [3],
+                         randSeed;
 #endif